Crystallography analysis of toughness in high strength ultra-heavy plate steel

et al. 2021

steel research int.

Herein, the start laminar cooling temperature to obtain different volume fractions of ferrite and bainite/martensite is optimized. Lower start laminar cooling temperature yields a higher volume fraction of ferrite and also decreases the transformation temperature of bainite/martensite. The yield strength of the experimental dual‐phase steel at 600 °C increases from ≈484.8 to ≈526.2 MPa by decreasing the start laminar cooling temperature from 650 to 600 °C. To understand the effect of microstructure, the dislocation density, volume fraction of precipitates, and Young's modulus at elevated temperature are statistically analyzed. A machine learning method, known as gradient boosting decision tree, is used to classify the grain boundaries. The statistical results suggest that the steel subject to lower start laminar cooling temperature shows higher Young's modulus and larger volume fraction of precipitates at elevated temperature. Furthermore, at elevated temperature, the boundary is stable and the proportion of Σ3 boundary is greater, when lower start laminar cooling temperature is adopted. Theoretical calculations results suggest that the lower transformation temperature of bainite/martensite improves the fire resistance of dual‐phase steels.

Section: Resultsmentioning

confidence: 99%

Effect of Microstructures on the Mechanical Behavior of Fire‐Resistant Dual‐Phase Steels at Elevated Temperature

Cong

et al. 2021

steel research int.

“…Thus, DBTT was determined by the density of high angle packet boundaries and block boundaries to a large extent. In one particular study, it was shown that V1–V2 variant boundary, which is one of the block boundaries, significantly influences toughness [ 34 ]. This is absolutely possible and reasonable because it is based on the premise that V1–V2 variant boundary dominates in quantity or the density difference of other types of boundary is small.…”

Section: Resultsmentioning

confidence: 99%

The Significance of Coherent Transformation on Grain Refinement and Consequent Enhancement in Toughness

Dong

et al. 2020

Materials

Coherent transformation is considered to be an effective approach to refine the microstructure and enhance toughness of structural steels. However, there are gaps in the knowledge on the key aspects of microstructure that govern toughness. In this regard, a low alloyed experimental steel with lean chemistry was subjected to a simple heat treatment involving austenitization at different temperatures, followed by quenching and tempering to obtain bainitic microstructures with different boundary composition. The microstructure of the four experimental steels was characterized by electron backscattered diffraction and mechanical properties were determined. The study indicated that the density of high angle grain boundaries does not adequately reflect the change of ductile-to-brittle transition temperatures (DBTT) of the experimental steels. Thus, we propose here a new mechanism on reducing DBTT from the perspective of misorientation of boundary, which takes into consideration these aspects in defining DBTT. One is inhibition effect on cleavage fracture by boundaries with high {100}-plane misorientation angles, and the other is ductility improvement by boundaries with high {110}-plane misorientation angles. Furthermore, the contribution of prior austenite grain boundary, packet boundary, block boundary, and sub-block boundary on toughness is also analyzed.

“…In addition, many grain and twin boundaries prevented crack propagation and enhanced fracture toughness. [32,33] Griffith's equation describes the effect of grain refinement on toughness, as shown in the following equation. [28] σ…”

Section: Effects Of Nitrogen On Mechanical Propertiesmentioning

confidence: 99%

Effects of Nitrogen on the Microstructure and Mechanical Properties of Fe–28Mn–10Al–0.8C Low‐Density Steel

Guo

Zhu

et al. 2021

steel research int.

Herein, the relationships among nitrogen content, inclusion characteristics, microstructure, and mechanical properties of low-density steel are investigated. The austenite grain size is refined with nitrogen addition to the low-density steel, and the ultimate tensile strength, plasticity, fracture toughness, and impact toughness are distinctly enhanced. However, the yield strength decreases due to a decrease in the ferrite content with nitrogen addition. AlN is determined to be the main inclusion in the high-nitrogen low-density steel, and the number percentage of AlN that is less than 2 μm reaches 84%, which promotes the refinement of austenite grains. In addition, aggregated AlN inclusions in the dimples of the steel are observed, which are generated during the smelting process, and the aggregated AlN inclusions tend to cause crack initiation in the low-density steel.